A system for detecting a mis-connected state between communication lines for a multi-type air conditioner and a method thereof. The system comprises an indoor unit pipe temperature detection unit for detecting a temperature of an indoor unit pipe, and a microprocessor for controlling an opening of an electronic expansion valve based on a temperature detected by the indoor unit pipe temperature detection unit, comparing a temperature response characteristic of the indoor unit pipe with a preset temperature response characteristic of a normal indoor unit pipe based on an opening of the electronic expansion valve, and thereby judging whether the communication lines are mis-connected to one another or not. An indoor unit from which the mis-connection has been generated is fast detected, and error information is displayed. Accordingly, the mis-connected state between the communication lines is restored, thereby preventing a damage of the system due to the mis-connection.